1. Statement of the Technical Field
The invention is directed to a satellite radio system. In particular, the invention is directed to a system and method for compensating for errors in the frequency of a transmitted signal caused by local oscillator variation and a shift in frequency due to the relative movement of a transmitter and a receiver.
2. Description of the Related Art
In wireless communication systems, including systems based on satellite communications, each of the devices connected to the communication system requires a stable local timing source. Generally, the timing sources in each of the devices must be synchronized in some way.
In a typical wireless communication system, an analog carrier signal is modulated by analog data or a digital bit stream containing information of interest to be transmitted. The carrier signal is typically a sinusoidal waveform at a frequency that is much higher than the frequency content of the information of interest. The carrier signal is typically generated using a local timing source in the transmitter. The modulated carrier signal is transmitted to the receiver.
The receiver in the wireless communication system demodulates the received signal to extract the information of interest. The receiver typically uses its own local timing source to demodulate the received signal. Any difference in frequency between the local timing source in the receiver and the local timing source in the transmitter will introduce a frequency error in the demodulated information of interest.
This problem is exacerbated when the transmitter and receiver are moving relative to one another. Such movement introduces an effect known as Doppler shift. Doppler shift is a perceived change in the frequency of a signal for an observer moving relative to the source of the signal. When the transmitter and receiver are moving towards one another, the frequency of the signal received at the receiver is higher, compared with the frequency of the signal emitted by the transmitter. Conversely, if the transmitter and receiver are moving away from one another, the receiver will receive a signal having a lower frequency than the signal emitted by the transmitter.
To reduce the effect of frequency errors, highly accurate timing sources may be used in each device connected to the wireless communication network. This technique reduces errors associated with differences in the frequency of the local timing sources included in each device. However, it is generally not practical to include such timing sources in every device connected to the communication network due to the high cost of highly accurate and stable local timing sources. Another option is to install highly accurate and stable timing sources in only some devices connected to the communication network. At least one of the devices containing a highly accurate and stable timing source is used as a reference timing source. Each of the other devices connected to the network uses a less expensive but tunable timing source, such as voltage controlled oscillator (VCXO) or a direct digital synthesizer (DDS). The low cost timing source, e.g., DDS, is tuned such that the local timing source is phase and frequency locked to the reference timing source.
A conventional satellite communication system employing Time Division Multiple Access (TDMA) includes a plurality of earth stations and a satellite transponder. The satellite transponder receives signals from the earth stations at one frequency, i.e., the uplink frequency, and retransmits the signals to the earth stations at different frequency, i.e., the downlink frequency. In a TDMA system, the earth stations have designated timeslots within a communications period called a frame in which to transmit a burst of information within each channel. In some cases, the frame is of such short duration that users transmitting low data rates, e.g., voice data, appear to receive continuous service.
One or more of the earth stations is designated as a reference station, which typically includes a highly accurate and stable timing source. The reference station produces a reference burst once per TDMA frame. The remaining earth stations, i.e., the traffic stations, use the information provided in the reference burst to synchronize their local timing sources to that of the reference station. For example, the receive timing of a traffic station may be modified based on the center frequency and arrival time of the reference burst.
Although this technique is effective when the transmitter and receiver are motionless with respect to one another, this is generally not the case in a satellite communication system. The reference burst received by each traffic station usually includes a Doppler shift caused by the daily movement of the satellite. The Doppler shift is significant when low earth orbits, medium earth orbits, and highly elliptical orbits are used. However, even a geostationary satellite will move with respect to the earth stations due to orbit imperfections. The Doppler shift will be different for each traffic station depending on the particular traffic station's position with respect to the satellite. Additionally, if the traffic station is portable, e.g., a hand-held radio, a Doppler shift caused by the movement of the traffic station will also be introduced.
There are generally strict tolerances with respect to the frequency error allowed to be introduced by a traffic station when transmitting to the satellite via an uplink frequency channel in a TDMA system. From the perspective of the satellite transponder, the signals received from each traffic station may have a different frequency when compared to the frequency of the signal produced by the reference station, i.e., the reference frequency. These frequency errors will propagate through to the particular downlink channel. The burst from each transmitting traffic station includes a preamble containing carrier and clock recovery sequence bits that can be used by the receiving traffic station to determine the center frequency and bit rate of the burst. The required length of the preamble is a function of the frequency uncertainty and other parameters, such as the signal-to-noise ratio at the receiving traffic station. As the frequency uncertainty increases, so does the length of the required preamble. Thus, the bandwidth efficiency of the TDMA system is a function of the acceptable transmit frequency error in the system.
The transmit frequency error, as seen by the satellite transponder, is a function of both Doppler shift and the error due to the differences between the frequencies of the local timing sources of the transmitting earth station and the reference station. For a traffic station to transmit a signal with minimal error, both the error introduced by the local timing source, and the error introduced by Doppler shift must be taken into account.
Thus, in a TDMA based satellite communication system, there is a need for a method for compensating for errors due to differences in the frequency of the local timing sources and Doppler frequency shift.